I think it all depends on how serious you want to get with maintenance - yes, mower engines are built to looser specs than car engines but they're also very durable as long as you give them even the minimum amount of care. Keeping oil level correct, sharpening blade, scraping grass off the deck, and keeping the air filter clean are all most consumers need to keep their engines running for a long time.
Case-in-point: my parents bought a Meijer brand mower with a B&S engine back in 1990, and it ran for 17 years without anything other than topping off the oil. In 2007 the air filter was cleaned (not even replaced), the spark plug changed, they bought a new blade, and they welded the deck back together. They still have that mower today, and it still runs (not perfectly, but pretty well given the circumstances).
Sometimes it's also luck of the draw. I knew someone who had a similar vintage mower who took it to a small engine repair shop to have it maintained and the engine failed on it sometime in the mid 2000's.
Bottom line - I wouldn't worry about it too much. And heck, if it breaks, it's a great opportunity to try out a manual push mower!